MULTIECHO IMAGING WITH SUBOPTIMAL SPOILER GRADIENTS

Although multiecho imaging may be used to measure transverse relaxatio n (T-2), B-1 and B-0 inhomogeneity generally gives rise to unwanted co herence pathway signals which result in erroneous T-2 measurements. On e approach to suppressing this unwanted signal is to center each rf re focusing pulse between spoiler gradients which dephase the unwanted si gnal; however, hardware limitations often dictate the use of suboptima l spoiler gradients, that is, gradients that cannot provide sufficient dephasing strength. Using simulations, this work demonstrates that by means of a small additional spoiler gradient prior to the first rf re focusing pulse it is possible to reduce substantially the contribution from unwanted coherence pathways in multiecho imaging studies that us e suboptimal spoiler gradients. This reduction of unwanted signal resu lts in measured T-2 values within approximate to 1% of values obtained using spoiler gradients of optimal strength. These results were found for a wide range of biologically relevant T-1 and T-2 values, missett ings of the rf refocusing pulse as large as 5%, and frequency offsets of up to 25 Hz. Multiecho image data agreed with the simulations. Usin g the additional spoiler gradient it is possible to reduce spoiler gra dient strengths by up to 75%. (C) 1998 academic Press.